As a control equipment for flow parameters on engine control, there are an ISC control equipment for controlling an auxiliary intake air amount to maintain an idling speed at a constant, an EGR control equipment for controlling an amount of circulation of exhaust gas to improve the fuel consumption or decrease Nox included in the exhaust gas, an evaporation control equipment for inhibiting the emission of the fuel evaporated from a fuel tank to air, and so on.
When the flow control equipment suffers a breakdown or when the flow control valve becomes out of order, an engine trouble occurs due to an abnormal increase in an idling speed, an occurrence of surge from instability of combustion, and abnormality (over-rich or over-lean) of air/fuel(A/F)ratio.
Especially, in order to address the emission regulation or the fuel-consumption regulation, a large amount of EGR and a large amount of evaporation-purge have been performed. Failures of their control units are directly relevant to engine stall, which not only hurts driver's feelings, but also very dangerous.
Accordingly, there are generally provided a control unit for preventing the out-of-control and another control unit for performing an evasion control (a fail-safe control) when a failure occurs. In the well-known technique, the safety can be maintained during running by fully closing a control system (ex. cutoff of an EGR pass) using, for example, an EGR ON-OFF valve (cut valve) when the failure is detected.
Further, in the prior art of the prevention of out-of-control in which a flow control valve driven by a motor is used, the function of fail-safe has not been considered. Therefore, such a failure sometimes occurs, in which an output signal is not provided to the motor. The setting time is provided only when a driving direction of a synchronous motor is switched in such a prior art.
Furthermore, as the setting time, different values are used according to a state of the variation in a motor torque, thereby preventing the deterioration of response.
When the control unit use not a cut valve, but an electromagnetic valve, the prior art equipment for evasion control is provided with two system of drive circuits for a flow control valve (if compared with an EGR valve using a step motor, a bipolar system). Therefore, when the flow control valve becomes not to generate the desired propelling force and it becomes difficult to control the flow control valve by using the drive circuit, such a flow control equipment can fully close the flow control valve by switching to the other system of drive circuit, thereby obtaining the desired propelling force necessary to drive the flow control valve. Such a flow control equipment is disclosed, for example, in Japanese Patent Laid-Open No. 4-301170. However, the drive circuit switching system is complicated and expensive.
There is known a method using a spring for returning the control valve, instead of the cut valve. Such a method is disclosed in Japanese Patent Laid-Open No. 1-316581.
The conventional return-spring method must have a strong return-spring which overcomes the friction of a screw and bulb sludge (which has an effect on the stick of the flow control valve), as a fail-safe function when the flow control equipment becomes abnormal. Therefore, there is a fear of out-of-control when the flow control valve is controlled , in addition to complicated mechanism and high manufacturing cost.
Further, while it is possible to prevent the deterioration of response by suppressing the control of the prevention of out-of-control to be the necessary and minimum amount, with maintaining the safety during running by preventing the out-of-control, the conventional return-spring method has no fail-safe function against the failure of the flow control equipment when the output signal is not provided to the motor.